METHOD TO MANUFACTURE AN RFID ANTENNA AND ANTENNA

The present invention generally relates to RFID (Radio Fre ^¬ quency IDentifier) manufacturing methods. The invention also relates to RFID antennas obtained by this method, as well as radio-frequency identifications devices using such antennas.

The use of a tag having an RFID antenna to identify and monitor objects is well known in the art. Such a tag comprises for instance an antenna circuit formed with conductive tracks, and an integrated circuit (IC) chip that includes a memory. It uses electromagnetic field produced by an RFID reader. If such a tag enters the magnetic field for a sufficient time, the RFID antenna will become energized and the electronic circuit can transmit a signal towards the reader or a separate receiving an- tenna.

A typical RFID tag, or inlay, generally includes an antenna and an integrated circuit (IC) chip connected to the antenna. The antenna pattern, also called later on antenna tracks, generally comprises a plurality of turns or loops that spiral around on a planar dielectric substrate and two loop ends, i.e. the antenna terminals. An antenna may comprise an integer number of loops so that the loop ends face each other with tracks in between as seen in FIG. 2. Other configurations are also possible. The antenna tracks can be either formed before being laid on the substrate or formed directly upon it (by etching or printing techniques for example), among known techniques.

The antenna inductance is essentially dependent upon the loop dimensions and, to a second extent upon the track cross section, while the antenna capacity is related to the spacing between loops.

For some applications of RFID antennas, it may be useful to vary the antenna resistance without modifying the inductance and capacity of the antenna significantly.

Therefore, the purpose of the present invention is to offer a manufacturing method for an RFID antenna that allows a control over the resistance with limited consequences on the inductance and capacity values .

Accordingly, the present invention provides a manufacturing method according to claim 1. The present invention further provides an RFID antenna according to claim 7, and an RFID tag according to claim 15. The invention takes advantage of the applicant finding that the first portion width can define the order of magnitude R of the antenna resistance, while varying the second portion width can allow a variation of the resistance value around the R value . Other features and advantages of this invention will further appear in the hereafter description when considered in connection to the accompanying drawings, wherein:

FIG. 1 illustrates an exemplary implementation of an RFID tag 1 according to the invention; FIG. 2 illustrates an RFID antenna used in the tag show in FIG. 1; and,

FIG. 3 illustrates a detailed view of FIG. 2. According to the example of FIG. 1, the tag 1 comprises a Epoxy Glass Tape (EGT) 2, 110 micrometer thick, over which a two layer track 3 is laid. Track 3 is made of a first copper layer over which a nickel layer is laid. The Cu layer is for example 35 micrometer thick while the Ni layer is 5 micrometer. An ultra thin gold layer may be laid over track 3. Substrate 2 may also be made of another suitable dielectric material, that is for ex- ample a polyamide or polyester, a plastic material, paper or cardboard, or any other dielectric material relevant for that matter .

As seen from FIG. 3, substrate 2 is obtained through cutting a rectangle in a reel 30 provided with a couple of rows of holes 20, the rows being parallel to each other and located clo ^¬ se to the opposed reel lateral edges. Holes 20 are used to drive the reel in a parallel direction to the rows main direction when manufacturing and handling the antennas for example.

Track 3 is made of a plurality of loops in the form of a spiral. The outer loop end, or outer terminal 4 is connected to a first contact end 5, while the inner loop end 6, as seen on FIG. 2 is connected to a second contact end 7. Track 3, also

called the antenna pattern, can be either printed (seπgraphy or any other suitable printing technique) , chemically plated or etched onto the substrate 2. The whole manufacturing process, including the here after method according to the invention, can be carried out on a reel as seen m FIG. 2. The turns are spread apart from each other by a certain gap, either of a constant value, or varying depending on the antenna characteristics.

Contact ends 5 and 7, e.g. in the form of small plates, are provided for the connection of an IC (integrated circuit as shown in FIG. 1) to the antenna.

According to the first implementation of the present invention, each loop comprises at least a first portion of a first width, and a second portion of a second width, said first width being distinct than said second width. As seen in FIG. 2, por- tions 10 present a smaller width compared to the wider portions 11.

Based on the Applicant's tests, the portions 10 with the smallest width Wl drive the level of the antenna resistance R close to the resistance value of a similar antenna with a unique track width corresponding to Wl. The portions 11 with the largest width W2 allow a variation of the antenna resistance. As the antenna inductance L is essentially dependent upon the loop dimensions and to a second extent the track cross section, and the antenna capacity C is related to the spacing between loops, us- ing the different portion widths allows to a certain extent to vary the resistance with limited impact on the other parameters L and C.

In an additional implementation of the present invention, first portions 10 extend in a first direction, while second por- tions 11 extend in a second direction, distinct from the first direction. In the illustration of FIG. 2, the directions are perpendicular to each other.

The antenna displayed on FIG. 2 is of a rectangular form. Each loop comprises a pair of first portions 10 parallel but re- mote from each other, and a pair of second portions 11 parallel but distinct from each other. The present invention is particularly well suited for antennas manufactured on a substrate in

the form of a reel 30 as seen in FIG. 2. Such a reel 30 presents at least one row of holes 20 extending in the reel longitudinal direction. In the illustration of FIG. 2, two rows of holes 20 are provided, respectively close to the reel 30 opposed lateral edges, i.e. one row 20' on the left hand-side of reel 30 and one row 20" on the right hand-side.

In an additional implementation of the present invention, first portions 10 extend close and parallel to each row of holes 20. The second portions 11 extend in the transversal direction of reel 30. As seen in FIG. 2, a plurality of first portions 10' is provided on the left hand side of reel 30 next to the left hand-side row 20' of holes, while a plurality of first portions 10" is provided on the right hand side of reel 30 next to the right hand-side row 20" of holes. Furthermore, when the antenna 3 presents a plurality of loops, first portions 10' may be distributing on each side of the left hand side row of holes 201, while first portions 10" may be distributing on each side of the right hand side row of holes 20".

Thanks to this disposition of first portions 10' and 10", optimal use of the reel surface may be achieved while taking into antenna pre-reguired dimensions and characteristics, espe ^¬ cially when specified inductance and capacity of the antenna are imposed. The invention allows marrying the use of an antenna substrate in the form of a reel with an increase control of the resistance in the context of pre-required antenna inductance and capacity.

As mentioned before, portions 11 with the larger width W2 allow tuning the antenna resistance value. In an additional implementation of the present invention, as seen in FIGS. 2 and 3, holes 12 are formed in the second portions 11 to vary the RFID antenna resistance. Holes 12 may have any suitable shape provided parts of the second portions 11 are removed. In the exemplary illustration of FIG. 3, holes 12 present the shape of a company logo. As seen in FIG. 2 and 3, the transversal direc- tion of second portions 11 is free of rows of holes 20 or any obstacle, and provides the required flexibility to adjust the antenna resistance.

Thanks to the antenna according to the invention, and to the related manufacturing method, one may vary the antenna resistance R by varying the second width W2 of the second portions 11, and as an additional tool, forming holes 12 in said second portions.

In the herebefore illustration, first portions 10 extend in the direction of the row of holes 20. The man skilled in the art can easily adapt the present teaching to an antenna wherein the second portions 11, with the largest width, extend in the direc- tion of row of holes 20.

An exemplary antenna according to the invention, and corresponding to the illustration of FIG. 2 would have the following characteristics :

- resonance frequency: 1 4.70 MHz; - parasistic capacity: 2.24 pF;

- AC resistance: 2.57 R,

- AC inductance: 1.65 μH.

As shown in Fig. 2, the antenna of rectangular form comprises three loops, namely an outmost loop, an innermost loop and an intermediate loop wound between the outmost and innermost loops .

Each loop 11' comprises a pair of first portions 10, having a first width Wl and wherein the length LlO of the first portions of each loop is different from one another. The first portions 10, representing the shortest sides of the rectangle, are parallel and opposite each other.

Each loop 11' further includes a pair of second portions 11, representing the longest sides of the rectangle, extending parallel and opposite each other. Said second portions have a second width W2 larger than the first width Wl. The length LIl of the second portions of each loop is also different from one another .

In the following table, gathered are the dimensions of each loop of the antenna (outmost, intermediate and innermost loops) , which enable to meet the above-mentioned antenna characteristics .

The dimensions are given as example only. In the present embodiment, said dimensions may vary within a range of ± lOOμm, more preferably within a range of + lOμm.

It should be noted that the lengths of the outer loop end 4 and the inner loop end 6 have little influence on the antenna characteristics, provided that said lengths represent preferably less than 1% of the overall length of said antenna.

The quality factor Q of the antenna is essentially proportional of 1/R. As mentioned before, a Ni layer is formed on track 3. This layer tends to increase the antenna resistance and hence decrease the antenna quality factor.